Hi new to the forum... I have done a search and could not find much on the subject, but I am sure it has been discussed before. I am from Melbourne Australia and have a 5.7 LS1 V8 GM powered vehicle, it has always been dealer serviced using 10w40 Valvoline (a bulk delivery variant the dealers use). I have just decided to use a local workshop, more convenient location, and I asked what oil they put in it, they said 15w40 Castrol Magnatec Professional, (once again a bulk delivery variant the workshops use), I am a little concerned that the 15w may be a tad thick at start up, seeing the trend these days seems to be heading towards 0w and 5w oils, the climate here is roughly from 2 degrees C (36F) in the coldest winter to maybe 41C (105F) in our hotest summer. What are your thoughts on the 15W or should I go back to the 10w which I would have to supply to the workshop as they only carry 0w40 5w40 10w30 15w40 and 20w50 weights. Appreciate your thoughts.